Cancers frequently exploit developmental genes/pathways to sustain their uncontrolled growth and invasion. To study the molecular mechanisms of prostate cancer (PCa) progression, we are focusing on the potential role of SOX9, an important developmental transcription factor. We have discovered that SOX9 is expressed in human fetal prostate epithelial cells and in the basal cells of adult prostate, suggesting it is involved in prostate organogenesis and homeostasis. Significantly, SOX9 is also expressed in primary and metastatic prostate cancer (PCa) in vivo and in PCa cell lines in vitro. Its expression is further increased in hormone refractory recurrent PCa, suggesting SOX9 is a potential biomarker for more aggressive PCa. SOX9 over-expression in LNCaP PCa cell renders cancer cells more efficiently establish in vivo xenografts and local invasion in mice. These xenografts accumulates extracellular matrix (ECM) and carry a vast vascular network, suggesting SOX9 promotes tumor growth by inducing tumor ECM production and angiogenesis. Microarray analyses of SOX9 regulated genes in response to SOX9 over-expression in LNCaP cells has selected several potential targets that are involved in cell proliferation, ECM formation and angiogenesis. Down-regulation of endogenous SOX9 by siRNA decreases cell proliferation and causes a cell cycle shift to G0/G1 in PCa cell lines and a reduced growth in PCa xenografts. Our studies further show that SOX9 expression in PCa cells is regulated by the Wnt/2-catenin, hedgehog (Hh), and androgen signaling pathways, suggesting SOX9 may be an important mediator of these critical pathways for prostate development and homeostasis, and for PCa growth and progression. These data suggest that SOX9, as a transcriptional factor, regulates genes (such as EMC proteins, proangiogenic factor or growth factors) expressed in basal cells that are critical to support the generation or survival of secretory epithelial cells in normal prostate. In PCa, dysregulated SOX9 expression allows the tumor cells to grow at local and metastatic sites in the absence of basal cell support. The overall goal of this proposal is to define the molecular mechanisms of SOX9 functions by identifying and validating SOX9 regulated genes or pathways in PCa cell lines, xenografts and patient samples. SOX9 function in PCa establishment and growth will be investigated through specific up- or down-regulating SOX9 expression in PCa cells and tumors. Finally, SOX9 and its target gene expression in PCa will be interrogated for its correlation with PCa establishment and progression as well as its potential as a prognostic biomarker. This study will not only provide insight in the molecular mechanisms of SOX9 functions in development and homeostasis of normal prostate, as well as in the tumorigenesis and progression of PCa. Public Health Relevance: We will study the functions of SOX9 in prostate development, maintenance and prostate cancer development and progression. Our results will lead to a better understanding of the molecular mechanisms of SOX9 function and identify potential targets for cancer therapy.